The present invention relates to pigment dispersions containing pigment, an aqueous carrier, and a pigment dispersant prepared by polymerizing (i) a prepolymer having a radically polymerizable group and (ii) a radically polymerizable monomer. The prepolymer is prepared by controlled radical polymerization, and has well defined polymer chain structure, molecular weight and molecular weight distribution. Monomer (ii) forms the polymeric backbone of the dispersant, and prepolymer (i) forms polymeric segments pendent to the backbone of the dispersant. The backbone of the pigment dispersant is hydrophobic, and at least a portion of each pendent polymeric segment is hydrophilic.
Pigmented coating compositions are used in a wide variety of applications including, for example, corrosion resistant primers and decorative topcoats in the automotive, industrial and appliance markets. The preparation of pigmented coating compositions generally involves mixing binder resin(s), crosslinker(s), additives, e.g., flow additives, and solvents with a compatible pigment dispersion. Pigment dispersions are typically prepared by mixing dry pigment with a pigment dispersant in the presence of a carrier medium, e.g., an aqueous carrier medium.
Dry pigments are available commercially in the form of agglomerated pigment particles. Pigment agglomerates are more likely to settle out of pigment dispersions and/or pigmented coating compositions, and are accordingly undesirable. To break the pigment agglomerates down into smaller agglomerates and/or individual particles generally requires the use of energy intensive mixing means (commonly referred to as grinding), e.g., sand mills and ball mills. During the grinding process the pigment agglomerates are broken down into smaller agglomerates and/or individual particles the surfaces of which are wetted by the pigment dispersant. The pigment dispersant suspends or disperses the pigment particles in the carrier medium and prevents their re-agglomeration on storage. It is desirable that the pigment dispersion remain substantially stable, e.g., showing minimal pigment settling and viscosity change with time, prior to its use in the preparation of a pigmented coating composition.
Reducing the environmental impact of pigmented coatings compositions, in particular that associated with emissions into the air of volatile organics during their use, has been an area of ongoing investigation and development in recent years. Accordingly, interest in aqueous pigment dispersions has been increasing due, in part, to the inherently low volatile organic content (VOC) of the aqueous pigmented coatings prepared therefrom, which can significantly reduce air emissions during the application process.
Pigment dispersants used in aqueous pigment dispersions are preferably compatible with both the aqueous carrier medium and the hydrophobic surfaces of the pigment particles. Such a combination of disparate properties can be achieved with a pigment dispersant having distinct hydrophobic and hydrophilic polymeric segments, i.e., having well defined polymer chain architecture. A wide variety of radically polymerizable monomers, such as methacrylate and acrylate monomers, are commercially available and can provide a wide range of properties including, for example, hydrophilic and hydrophobic properties. The use of conventional, i.e., non-living or free, radical polymerization methods to synthesize pigment dispersants provides little control over molecular weight, molecular weight distribution and, in particular, polymer chain structure. Aqueous pigment dispersions containing pigment dispersants prepared by conventional non-living radical polymerizations, typically have poor stability, e.g., the pigments re-agglomerate and/or settle out of the dispersion, and are unsuitable for the preparation of aqueous pigmented coatings compositions.
The continued development of aqueous pigment dispersions that are stable and suitable for the preparation of aqueous pigmented coating compositions is desirable. In particular, it would be desirable to develop aqueous pigment dispersions that comprise pigment dispersants having well defined polymer chain architecture in which distinct hydrophilic and hydrophobic polymer segments are present.
U.S. Pat. Nos. 5,807,937, 5,789,487 and 5,763,548, and International Patent Publication Nos. WO 98/40415, WO 98/01480, WO 97/18247 and WO 96/30421 describe a radical polymerization process referred to as atom transfer radical polymerization (ATRP). The ATRP process is described as being a living radical polymerization that results in the formation of (co)polymers having predictable molecular weight and molecular weight distribution. The ATRP process is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). The ""937 and ""548 patents also describe (co)polymers prepared by ATRP, which are useful in a wide variety of applications including, for example, dispersants and surfactants.
In accordance with the present invention, there is provided a pigment dispersion comprising:
(a) pigment;
(b) an aqueous carrier selected from water and a mixture of water and at least one organic solvent; and
(c) a pigment dispersant prepared by polymerizing a composition comprising,
(i) at least one prepolymer having a radically polymerizable group, said prepolymer being prepared by controlled radical polymerization of at least one radically polymerizable ethylenically unsaturated first monomer, said prepolymer having a polydispersity index of less than 2.0; and
(ii) at least one radically polymerizable ethylenically unsaturated second monomer to produce a polymeric backbone segment in said pigment dispersant; wherein said prepolymer after polymerization with (c)(ii) is in the form of polymeric segments pendent to said polymeric backbone segment, at least a portion of each pendent polymeric segment is hydrophilic, and the polymeric backbone segment is hydrophobic.
The features that characterize the present invention are pointed out with particularity in the claims which are annexed to and form a part of this disclosure. These and other features of the invention, its operating advantages and the specific objects obtained by its use will be more fully understood from the following detailed description and the accompanying drawings in which pigment dispersants useful in embodiments of the invention are illustrated and described.
Other than in the operating examples, or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc, used in the specification and claims are to be understood as modified in all instances by the term xe2x80x9cabout.xe2x80x9d